1. Technical Field
The present invention relates to a recorder/reproducer and a recording/reproducing method for an optical information recording medium for optically recording and/or reproducing information, and more particularly, to focus control of a recorder/reproducer using near field light.
2. Background Art
There is proposed a technique using near field light as a system capable of recording and reproducing data into/from an optical disk with high density. It is an optical system combining a condensing lens and solid immersion lens (SIL) that is attracting attention as a condensing optics for producing near field light in recent years. This combination can realize a numerical aperture higher than the NA (numerical aperture) of the condensing lens. Increasing the numerical aperture of an optical system reduces the spot size and can thereby realize higher density recording.
Since the optical system using the SIL needs to cause light exuding from the exit surface of the SIL to be incident on the surface of an optical disk, the optical system is required to extremely shorten the distance between the SIL and the surface of the optical disk. In an optical system such as a DVD, the distance between the objective lens and the surface of the optical disk is approximately 1 mm, while in the case of the SIL, the distance between the SIL and the surface of the optical disk needs to be reduced to approximately 100 nm or less. Furthermore, when the distance between the SIL and the surface of the optical disk varies, near field light may not be obtained or the SIL may collide with the optical disk. Therefore, such control is also required that keeps constant the distance between the SIL and the surface of the optical disk.
There is proposed a method using an air slider to realize such control. This method uses a configuration with a SIL mounted on a slider. That is, when an optical disk rotates, an air flow is produced between the surface of the optical disk and the slider and the air flow makes the slider float and thereby keeps constant the distance between the SIL and the surface of the optical disk.
Furthermore, a method called a “gap servo” is proposed as another control method. This method is disclosed, for example, in Patent Document 1. This method detects the quantity of reflection light from an optical disk based on near field light, adjusts the positions of the condensing lens and SIL in the optical axis direction so that the light quantity approximates to a constant value to thereby control the distance between the SIL and the surface of the optical disk (that is, the gap).
However, the above described conventional method using an air slider has the following problems.
Control over the air slider is merely passive control that is flotation by an air flow and never actively controls the position of the SIL. Therefore, when, for example, locally varying irregularities exist on the surface of the optical disk, the position of the SIL cannot follow the variation, which may cause the SIL to collide with the surface of the optical disk. On the other hand, attempting to prevent locally varying irregularities on the surface of the optical disk requires the surface to be mirror-finished after the optical disk is manufactured, which constitutes a factor of increasing the manufacturing cost.
Attempting to control the air flow so that the slider follows the above described varying irregularities as much as possible may complicate the design of the slider shape. Furthermore, not only varying irregularities of the surface but also warpage of the optical disk itself (called “tilt”) constitutes a factor of reducing the tracking capability of the SIL. Furthermore, when the number of revolutions of the optical disk is changed, the air flow changes, which also results in a problem that the gap deviates from a desired distance.
The above described conventional method using the air slider as well as the above described conventional method using the gap servo involve the following problems.
A first problem is that when the distance between the optical disk surface and an information layer where information is actually recorded is not constant, a defocused state may occur with respect to the information layer. Inserting a protective layer between the information layer and the surface of the optical disk for the purpose of protecting the information layer may vary the distance between the surface of the optical disk and the information layer due to variations in thickness of the protective layer produced in manufacturing steps. In such a case, although the gap can be kept constant, the state of focusing on the information layer cannot help but be changed.
A second problem is that there may be a problem in recording and reproducing data into/from a disk with a plurality of information layers provided in the optical axis direction, a so-called multilayered disk. That is, in the multilayered disk, the distance of each information layer from the surface of the disk varies from each other, but according to the conventional method, it is indeterminate at what distance from the surface of the disk laser light is focused when the gap servo is operated. Therefore, when the gap servo is operated, if laser light is focused on any layer of the information layers by accident, information recorded in the information layer may be mistakenly erased or destroyed and such a probability increases as the number of information layers increases.    Patent Document 1: Japanese Patent Laid-Open No. 2001-76358